The present invention relates to a photoconductor for electrophotography (hereinafter "photoconductor") for use in electrophotographic apparatuses such as printers and copying machines.
A basic photoconductor structure includes an electrically conductive substrate (hereinafter "substrate") and a photosensitive film on the substrate. The photosensitive film may include a charge generation layer and a charge transport layer. An undercoating film is interposed between the substrate and the photosensitive film to cover surface defects on the substrate, facilitate obtaining a uniform photosensitive film, improve adhesiveness between the substrate and the photosensitive film and prevent charge injection from the substrate to the photosensitive film. The undercoating film is made of resin or contains inorganic pigment dispersed into binder resin.
In addition to the previously described requirements, the undercoating film must also exhibit electrical resistance low enough to avoid adversely affecting the electrophotographic properties of the photoconductor structure. The undercoating film should not change charging potential, residual potential or sensitivity when the photoconductor is used repeatedly or when subject to environmental conditions. Environmental conditions that adversely affect electrographic properties include a low temperature/low humidity environment, or, when the photoconductor is used in a high temperature/high humidity environment.
Inorganic pigments are used to either prevent laser beam interference or to adjust the electrical resistance of the undercoating film. However, when inorganic pigment is dispersed into the undercoating film, aggregation of the pigment occurs. This in turn causes defects such as concave and convex portions and pin holes in the undercoating film. The defects prevent formation of an undercoating film that is adequately uniform.
The undercoating film must exhibit an appropriate breakdown voltage to avoid image defects due to dielectric breakdown which occurs when the so called contact charging method is employed. When the contact charging method is employed, the photoconductor is charged by applying a voltage directly onto the photoconductor surface.
Resin undercoating films which contain no additives have been considered for use in a photoconductor structure. Preferable conventional resins for the undercoating film include acrylic resin, polyamide resin, vinyl chloride resin, vinylidene chloride resin, polycarbonate resin, poly(vinyl alcohol) resin, phenolic resin, polyurethane resin, and polyimide resin. When these resins are used without additives, they have high electrical resistance. The high electrical resistance causes a lowered sensitivity and a rise in residual potential in the photoconductor. The lowered sensitivity and rise in residual potential cause low image density and fog (stained background). Tremendous sensitivity lowering and rise in residual potential result when the undercoating film is thick enough to completely cover the surface defects on the substrate or to adjust the breakdown voltage. This is especially true when the photoconductor is used in a low temperature and low humidity environment. Therefore, a resin undercoating film which does not contain an additive is not practical for use in a photoconductor structure.
Alternatives have been proposed for adjusting the electrical resistance and for obviating the foregoing problems. These alternatives include the addition of an additive to the undercoating film. These additives include a filler containing metal powders such as Al and Ni, addition of conductive metal oxide such as indium oxide, tin oxide and zinc oxide or addition of carbon black. However, it is difficult to uniformly disperse fillers such as metal power and conductive metal oxide into the undercoating film. Aggregation of the filler causes defects in the coating film.
Another proposed solution to the above problems is to add so-called low-molecular-weight-type surface active agents to the undercoating layer. These surface active agents include: non-ionic surface active agent such as poly(oxyethylene alkylether) and glycerol fatty acid ester, an anionic surface active agent such as sodium alkylsulfonate, and a cationic surface active agent such as tetaalkylammonium salt. In doping the low-molecular-weight-type surface active agent in the undercoating film, surface defects are often caused due to bleeding out (segregation) of surface active agent during the formation of the undercoating film. The surface active agent is so hygroscopic that the undercoating film containing the surface active agent is adversely affected by environmental changes, especially changes in humidity.
In making a photoconductor which includes a substrate, an undercoating film on the substrate and a photosensitive film on the undercoating film, the photosensitive film is usually formed by dip-coating or by spray-coating. Some types of solvents contained in the photosensitive film coating liquid can dissolve or otherwise transform the undercoating film. The resulting uneven and non-uniform coating film causes irregular distribution of the potential and degraded image qualities.